1. Field of the Invention
This invention relates to a light transmitting/receiving module, eg., to PD/LD(or LED) module having a light signal receiving part and a light signal transmitting part. This invention, in particular, proposes an improvement of an inexpensive resin-molded package for a PD/LD module for simultaneous bidirectional optical communication by light signals of different wavelengths. This invention aims at the protection of the PD port against the noise generated by the LD port in the simultaneous, bidirectional PD/LD module.
2. Description of Related Art
This application claims the priority of Japanese Patent Application No. 11-30855 (30855/1999) filed on Feb. 9, 1999 which is incorporated herein by reference.
A prior PD/LD module has an independent laser diode (LD) module (e.g., shown in FIG. 1) and an independent photodiode (PD) module (e.g., shown in FIG. 2). The PD module is sealed in a metal package. The LD module is also sealed in another metal package. The PD module and the LD module are independently shielded from noise by their own metal-can package.
A prior typical LD module is explained by referring to FIG. 1. The LD module 1 has a round metal stem 2. A pole 3 stands on the stem 2. An LD chip 4 is mounted on a submount fixed on the side wall of the pole 3. A PD chip 5 is fitted at the center of the stem 2 just beneath the LD chip 4. The stem 2 holds a metallic cylindrical lens holder 6 welded on the upper surface. The lens holder 6 sustains a lens 7 at a top opening. A conical ferrule holder 8 is welded upon the lens holder 6. A ferrule 10 holds an end of an optical fiber 9. The ferrule holder 8 supports the ferrule 10 at a top opening. A cap 14 having a top hole covers the central part of the stem 2.
Edges of the ferrule 10 and the fiber 9 are slantingly ground for preventing the reflecting light from going back to the LD 4. The LD 4 yields transmitting light signals. The rear PD 5 is equipped for monitoring the output light power of the LD 4. The transmitting signals are converted from electric signals to light signals by modulating the driving current of the LD 4. The driving current of the LD 4 and the photocurrent of the PD 5 are supplied from or replenished to external electric circuits through lead pins 11, 12 and 13 on the stem 2. The stem 2, the lens holder 6 and the ferrule holder 8 are made from metals. These members form a metal package of the LD module. Since the package is made from metals, the package shields and protects the LD module from external electromagnetic noise. The transmitting part generates large electromagnetic waves (inner noise) due to the big driving current flowing in the LD. The inner noise is shielded by the LD metal package. The PD module was doubly protected from the noise by the metal-can package.
A prior PD module is explained by referring to FIG. 2. The PD module 15 has a circular metallic stem 16 including a central protuberance. A PD chip 17 is mounted upon the protuberance. A metal cap 18 with a top opening is welded upon the stem 16 for protecting the PD chip 17 and other members. A metallic cylindrical lens holder 19 is welded upon the stem 16. The lens holder 19 maintains a lens 26 at a top opening. A conical ferrule holder 20 is welded upon the lens holder 19. A tubular ferrule 22 keeps an end of an optical fiber 21. The ferrule 22 is inserted and fixed in a top hole of the ferrule holder 20. Ends of the fiber 21 and the ferrule 22 are ground slantingly for inhibiting the end surface from reflecting the light back to the laser. The PD module 15 is protected in a metallic case comprising the metal stem 16, the metal lens holder 19 and the metal ferrule holder 20. The receiving part has a high impedance which is subject to external noise. The metallic case connected to the ground level shields the PD module from the external noise.
A driving circuit accompanies the LD module for supplying the driving current modulated by the transmitting signals. The driving circuit contains a modulating circuit, a power amplifier or so. The driving circuit is enclosed by a metal package which forbids noise to go out of the driving circuit. Thus, the prior driving circuit does not emit noise. An amplification circuit follows the PD module for amplifying the photocurrent. The amplification circuit is also resistant against external noise since it is protected by a metallic case.
The prior PD/LD module has an independent PD module, a free amplification circuit, an isolated LD module and a separated driving circuit. Each of the parts is stored in an independent metallic package. The metallic packages kill mutual interference between the LD and the PD. Simultaneous signal reception and signal transmission does not induce a problem that the strong transmitting signals from the LD would mix with the weak received signals on the PD in the prior PD/LD module. The simultaneous transmission and reception is an important precondition of the present invention. The LD generates strong electromagnetic waves since the LD is applied by strong, rapid-changing modulating current. The reception port is subject to noise due to the weak photocurrent and the high amplification rate. The simultaneous transmission and reception has a tendency of bringing the transmission signals to the reception port as noise. If the transmitting part and the receiving part are stored in two different metal cases, the packages doubly shield and protect the PD port from the noise of the LD.
The noise (crosstalk) transference from the LD to the PD is an important criterion of the PD/LD module which makes use of a single fiber for transmitting and receiving signals, as shown in FIG. 3. FIG. 3 shows a bidirectional system between the central base station and the subscriber (ONU: optical network unit). Although there are a plenty of subscribers for a single station, FIG. 3 shows only a single subscriber (ONU). The base station is equipped with an LD1 for transmitting optical signals to the ONU and a PD1 for receiving signals from the ONU. The LD1 and the PD1 are connected by fibers 27 and 33 to a WDM (wavelength division multiplexer) 28. The WDM 28 joins with an optical fiber 29. The ONU has an LD2 for transmitting signals to the station and a PD2 for receiving signals from the station. The PD2 and the LD2 are connected by fibers 31 and 32 to another WDM 30. The ONU side WDM 30 communicates with the station side WDM 28 by the fiber 29. In FIG. 3, the signals propagating from the station to the subscriber are called "down-flow". The signals spreading from the ONU to the station are called "up-flow". The single fiber 29 allows simultaneous bidirectional transmission of information between the ONU and the station.
The transmission/reception system has a simultaneous communication type and an alternate communication type concerning the timing of signal transmission, and has a single fiber type and a double fiber type with regard to the number of fibers. The alternate type is also called a ping-pong communication. There are thus four types of communication systems. The system of FIG. 3 can be applied also to the alternate type. The alternate (ping-pong) type allocates different timing for the transmission and the reception on the subscriber site (ONU). The alternate type allows both the single-fiber medium and the double-fiber medium. The ping-pong type system can make use of the same wavelength of light for both the up-flow and the down-flow transmissions. In the case of using the same wavelength, the light division devices 28 and 30 are beam-splitters. The ping-pong type is free from the crosstalk or the internal noise between the transmitting signals and the receiving signals, since the timing of the reception is different from the timing of the transmission on the subscriber site. There is no need for shielding the receiving part from the transmitting part in the ping-pong type. Thus, the ping-pong type is excluded from the scope of the present invention.
The simultaneous type is the subject of the present invention. FIG. 3 can be also applied to the simultaneous communication. Since the system relies only upon the single fiber, different wavelengths (.lambda.1 and .lambda.2) should be allocated to the down-flow (.lambda.2) signals and the up-flow (.lambda.1) signals. In the multiwavelength communication, the light division devices 28 and 30 mean WDMs (wavelength division multiplexers) which have wavelength selectivity.
FIG. 4 shows the structure of a prior ONU (subscriber site) module. The module has an LD module 1 and a PD module 15 which are contained in separate independent packages. For example, the up-flow signals take 1.3 .mu.m light (.lambda.1) and the down-flow signals adopt 1.55 .mu.m light (.lambda.2). A single optical fiber 34 carries the down-flow and the up-flow signals between the base station and the subscriber site. What integrates and divides the down-flow and the up-flow signals by the difference of the wavelengths is a WDM. FIG. 4 denotes an optical waveguide type WDM 37. Two waveguide paths exchange their energy at a joining part 38 fabricated on the waveguide layer. In the example, the 1.3 .mu.m light goes straight but the 1.55 .mu.m light bends at the joining part 38. The 1.3 .mu.m up-flow signals emanating from the LD module 1 pass a fiber 9, an optical connector 41, a fiber 36, the WDM 37, an optical connector 35 and the fiber 34 to the base station.
The 1.55 .mu.m down-flow signals propagate in the fiber 34, the optical connector 35, the WDM 37, a fiber 39, an optical connector 40 and a fiber 21 and enter the PD module 15. In this case, the single fiber 34 can be replaced by two fibers. Despite the number of the fibers, there is a probability of the transmitting part having an influence upon the receiving part through electromagnetic waves. The problem of the crosstalk or the inner noise is surely denied by building the LD module sealed by a metal package as shown in FIG. 1, the PD module sealed by an independent metal case as shown in FIG. 2. The expensive LD/PD module of FIG. 4 is immune from the noise, since the independent metal case shields the PD chip electromagnetically and the metal LD case suppresses the electromagnetic waves from the LD chip.
The ONU device of FIG. 4 employing the metal-shielded LD module of FIG. 1 and the metal-case PD module of FIG. 2 is free from the problem of the inner noise of the LD (transmitting part) invading and disturbing the PD circuit (receiving part). The LD/PD (ONU) module of FIG. 4, however, is too expensive owing to many costly parts and devices. Furthermore, there is another defect that the direction of the fiber is perpendicular to the plane of the stem (package). This is a geometric defect which induces a difficulty for reducing the size of the module. Since signal light propagates for a long distance in space, the LD and the PD require lenses for converging the light. The PD module and the LD module are bulky and large by themselves. Then the PD/LD module integrating the PD module of FIG. 2 and the LD module of FIG. 1 should be a large device. The expensive metal packages for the PD, the LD, the LD driving circuit and the preamplifier raise the parts cost of the ONU module of FIG. 4. As long as the ONU module persists in the expensive large-sized LD/PD module, the bidirectional communication would not prevail in the general public. Inexpensiveness and small-size of the ONU modules are the requisite for prevalence of the optical bidirectional communication. Miniaturization of the ONU (LD/PD) module dislikes separate PD and LD which would raise the cost and the size.
Some trials for reducing the cost and the size of the ONU module have been done by unifying a PD and an LD (and an amplifier) on a common substrate. The new contrivance tries to simplify the module by mounting an LD and a PD on a single substrate and guiding light in parallel to the substrate. The mode of mounting is called planar lightwave circuit (PLC) mounting since the LD, the PD and the waveguides are positioned at the same height on the substrate. The planar module has horizontal propagating directions of the PD light and LD light different from FIG. 1 and FIG. 2. Besides the PD and the LD, an amplifier AMP is often mounted on the same substrate. In the case, the resistance against noise is raised by the amplifier stored in the same package.
The common substrate containing the LD, the PD and the AMP diminishes the size of the module and reduces the weight. The chips of PD, LD and AMP are small enough. The module containing the chips is also small. The number of the packages is reduced. The reduction of the packages decreases the cost of the module. Adoption of a ceramic package would lead to an expensive module despite the reduction of the number of the packages. A resin mold type gives the most popular and most inexpensive packages. This invention aims also at sealing the module with a resin mold type package.
Even the planar type module is free from the problem of noise when the LD driving current is weak enough (namely the short-range communication). The large driving current required by the long-range communication induces a difficult problem of the crosstalk (or inner noise) from the LD to the PD. Strong LD electric signals propagate in space and induce noise in the receiving part. High repetition rate and strong power of the LD signals yield strong electromagnetic waves. The receiving part of the PD and the AMP has a high input impedance and high amplification ratio which have a tendency of inviting noise. Namely, the LD/PD module has a structure which allows the receiving (PD) part to couple with the transmitting (LD) part electromagnetically. The noise transmission from the LD to the PD is also called "crosstalk". The noise lowers the sensitivity of the receiving part. The shorter distance between the LD and the PD/AMP brings about the more serious difficulty. Namely, the problem of the crosstalk would become more severe, when the size of the module is reduced.